The invention pertains to a traction device provided with a direct current, series excited motor, the power supply of which is controlled by a direct current static converter of the chopper type. Devices of this kind may be used in vehicles with batteries (wagons, automobiles, etc.) for example, and more specifically for traction of electric locomotives.
The choice of the motor of the series type, in the field of traction, has been desirable for a long time because of its numerous advantages compared to other known types. In addition, the control equipment for such motors, which was electro-mechanical, has developed into static type equipment using direct current converters of the chopper type. Indeed, these converters enable a variable voltage to be supplied in a continuous manner, offering numerous advantages compared to a device of the rheostat type, for example. A chopper may be compared to a static contact breaker with periodic on and off modes for transmitting an average variable voltage. In addition, a slightly oscillating current is generally desirable.
This is why a circuit called a free circulation circuit, or free wheeling circuit, for maintaining the current in the motor when it is interrupted by the chopper, has been utilized. The self-inductance of the motor circuit accumulates energy when the chopper is conducting and releases this energy through the free wheeling circuit when the chopper is non-conducting. Such a free wheeling circuit is easily obtained by means of a diode in parallel with the motor. In addition, a smoothing self inductance is generally included in the circuit in order to reduce the oscillation of the current.
If T represents the length of one period of the chopper, and tf the duration of conduction of the chopper during a period, it is plain that the ratio of conduction tf/T of this chopper is adjustable by varying either tf (fixed frequency variation) or T (variable frequency variation).
Thus, traction devices with a direct current series type motor that is fed by a static chopper generally include a free wheeling diode and a smoothing self-inductance. However, it is known that for this kind of device, in which the inductor includes a thyristor shunting device whose control is linked to the chopper's, or in which the series inductor is connected in the circuit of the free wheeling diode (automatic shunting of inductor), it is not possible to operate the chopper under continuous conduction without taking the chance of entirely cancelling the flux of the motor and thereby creating a dangerous mode of operation.
Such devices have, in particular, the double disadvantage of creating permanent losses because of the chopper operation, on the one hand, and, on the other hand, to cause an unwanted drop in the voltage between the power supply system and the motor, which reduces the performance of the device and its yield after the starting phase.
There are also devices in which the chopper may be forced or enabled to operate under continuous conduction after the starting phase, the motor including then two series type field coils. One of the coils is in the circuit of the free wheeling diode and the other is in the armature circuit of the motor. However, devices of this type have the disadvantage of requiring an uncontrollable variable starting effort.
It is therefore, a general object of the present invention to provide a device that overcomes the above mentioned disadvantages. In addition, the device, according to this invention, increases the stability of the traction system, to reduce the oscillation of the field flux of the traction engine and to improve the yield.
In order to achieve these goals, this invention proposes a device for electric traction equipped with at least one d.c. motor, the field coil of which is in series with the armature coil and the smoothing coil, and the power supply of which is adjusted by means of a converter of direct current of the chopper type. The inductor also includes a shunting device for reduction of excitation and a free wheeling diode is in parallel with the motor and the smoothing coil. The field coil is divided into at least two partial coils which are in series, in order to generate additive flux, one of them only being provided with the shunting device.
The chopper is preferably of a type which may be operated under continuous conduction. In this case, when the chopper is forced into continuous conduction, it is, of course, possible to regain control of the total field flux by controlling the operation of the chopper.
However, one particular embodiment of the invention is advantageous in that it includes a means for adjusting the total field flux in addition to adjusting the chopper in order to provide an adjustment of the excitation rate when the chopper is forced into continuous conduction. In this case, the means for adjusting is advantageously made up of the shunting device of the corresponding partial field coil, this device being then adjustable independently of the chopper. For this purpose, the shunting device may include an auxiliary chopper, for example, or a plurality of resistances set in parallel, each one being provided with an individual circuit breaker.